The present invention is directed towards new and entirely unexpected urea fertilizers having reduced ammonia volatilization. The present fertilizers are in a granular form and contain a central fertilizer particle of ammonia volatilization inhibitor. The inhibitor reduces ammonia volatilization resulting from the break down of urea when urea fertilizer granules are applied to soil.
From an economical and environmental stand point it is becoming increasingly important to improve efficiency of nitrogen delivery to plants from fertilizers. One method to improve nitrogen delivery efficiency is to reduce nitrogen losses due to volatilization. Urea, CO(NH2)2, is a white crystalline solid containing 46% nitrogen and is widely used in the agricultural industry as a fertilizer. Volatilization of ammonia occurs when urea is broken down in the soil. In order for plants to absorb nitrogen from urea, the urea must first be chemically decomposed as follows:CO(NH2)2+H2O+urease→NH3+H2NCOOH→2NH3(gas)+CO2(gas) 
Urease is an enzyme that catalyzes the hydrolysis of urea, ultimately into carbon dioxide and ammonia. Urease is most commonly found in bacteria, but also in fungi such as yeast and several higher plants.
Thus in soil, urease is a naturally occurring microbe enzyme that catalyzes the hydrolysis of urea to carbamic acid (H2NCOOH). Carbamic acid is unstable. Decomposition of carbamic acid occurs without enzyme catalysis to form ammonia and carbon dioxide. Ammonia will be volatilized or released to the atmosphere unless reacted with water as follows:NH3(gas)+H2O→NH4++OH−
The present invention is believed to inhibit ammonia volatilization from urea by several mechanisms, but important mechanisms are believed to be the inhibition of urease producing microbes, or by one mechanism or another, interference with urease activity.
Ammonia losses can be reduced when a urease inhibitor is applied with or within a urea fertilizer. There are several known approaches to employing fertilizers and reduce ammonia losses. One approach employs the inhibitor, calcium cyanamide, as in the product, Stabl-U™, made by Bi-En Corp and described in U.S. Pat. No. 6,576,035, which comprises calcium cyanamide particles that are coated with urea. Another approach uses the most common inhibitor, NBPT (N-[n-butyl]thiophosphoric triamide), sold under the trade name Agrotain®. This product is applied as a coating to the outside surface of urea particles which prevents urease enzyme from breaking down urea for up to 14 days.
Boric acid and other boron compounds have been used as urease inhibitors for reducing ammonia volatilization of urea fertilizers (U.S. Pat. No. 3,565,599; U.S. Pat. No. 3,523,018; and U.S. Pat. No. 6,830,603). A coating product has been commercialized from these patents, produced by Weyerhaeuser, called Arborite®. Arborite is a reacted boric acid coating that may be used to coat the outside surface of fertilizer particles.
The melting point of urea is 270-275° F. A processing problem that is associated with combining urea and boric acid is that when boric acid is heated to a temperature equal to or greater than 158° F., boric acid starts to melt and decompose as follows:H3BO3→HBO2+H2O
The water formed by the above decomposition can cause process and product problems. The presence of high moisture content in the product urea melt may be detrimental to a urea granulation process such as causing unwanted particle agglomeration and dust formation. Product quality is diminished due to low particle strength. Storage and handling properties will be undesirable due to high moisture, and low particle strength leads to caking in bulk piles or bags.
Urea containing boric acid has a lower critical relative humidity of 50% compared to urea alone which has a critical relative humidity of 72%. This means that urea having a boric acid based coating or urea mixed with boric acid will absorb moisture from the atmosphere at a lower humidity than urea alone.
In the prior art there are other known techniques used for applying boric acid to fertilizer granules. These techniques include dissolving boric acid into water, or reacting boric acid with amino alcohols, and then apply the resulting fluid to the outside surface of the urea granules. This method is disadvantageous because it requires an additional drying step. Further, particle surface area varies with particle size (proportionately less surface area as particle diameter increases) and thus limits the proportional amount of boric acid that can be applied to the surface. Surface application is also limited to the amount that will adhere to the surface. From a manufacturing viewpoint, surface application disadvantageously exposes manufacturing personnel to boric acid dust generated by handling.
Another method for incorporating boric acid into a urea containing fertilizer granule is to add these components to the urea melt prior to granulation resulting in a homogenously mixed fertilizer granule. A disadvantage of adding boric acid to the melt is that dust generated during the manufacturing process will contain boric acid and potential personnel exposure to the dust. Inhalation is the most significant route of exposure in occupational settings.
Due to these aforementioned potential problems, it is believed that these types of products were never produced commercially.